During the recent decade an ever growing application is enjoyed by electrodes comprising a metal base with deposited thereonto a thin coating of a compound possessing electrocatalytic properties.
Thus, known in the art are electrodes comprising a current-conducting substrate of titanium, niobium, tantalum, zirconium and a coating deposited thereonto and resistant against the electrolyte and the electrolysis products; said coating consists of a mixture of one or more oxides of film-forming metals such as aluminium, tantalum, titanium, zirconium, niobium, bismuth and tungsten with one or more metals such as palladium, platinum, rhodium, iridium, ruthenium, osmium, gold, silver, iron, nickel, chromium, lead, copper, manganese, oxides of these metals, nitrides, carbides sulphides thereof and their mixtures as well (cf. USSR Inventor's Certificate No. 369923).
These electrodes have substantial advantages over the prior art graphite electrodes. The principal advantages of the metal-oxide electrodes over graphite ones reside in the following:
1. a considerably longer service life of electrodes;
2. stable dimensions of electrodes excluding the voltage increase with time during electrolysis in processes with a solid cathode and avoiding the need in adjusting the electrode position to maintain a constant voltage in electrolyzers with a mercury cathode;
3. the absence of slime contaminating the membrane.
However, despite the above-mentioned advantages, a wide application of metal-oxide anodes is restricted first of all by a high production cost thereof as compared to that of graphite electrodes.
An essential disadvantage of metal-oxide anodes resides also in a high sensitivity thereof to shortings which restricts a broad application thereof in electrolyzers with a mercury cathode.
For this reason, numerous attempts have been taken to develop graphite electrodes possessing improved operation characteristics.
Known in the art are graphite electrodes impregnated with different electrochemically inert organic substances such as products of polymerization of oils (cf. Canadian Pat. No. 602053), polyester resin (Czechoslovakian Pat. No. 95463), allyl resins (Japanese Pat. No. 48-15149, Cl.13/7 D 131), products of polymerization of tall stand oil (cf. USSR Inventor's Certificate No. 167832).
In practicing of graphite electrodes impregnated with electrochemically inert organic substances as anodes for chlorine electrolyzers, the anode stability is increased by not more than 1.5 times as compared to anodes from a non-impregnated graphite.
A disadvantage of these anodes is in limited allowable operating current densities (for example, not more than 1.5 kA/m.sup.2 in the production of chlorine with a solid cathode and not more than 8-9 kA/m.sup.2 in the production of chlorine by processes with a mercury cathode). At higher current densities, an accelerated destruction of the anode is possible due to a surpassed critical swelling potential. The reason for limitation of a working current density of such electrodes resides in a higher potential of the anode after impregnation thereof with an inert organic substance. For the electrode impregnated so that all its pores are totally closed (which is most advantageous from the standpoint of lowering the inside wear), the permissible current density is substantially lower than the one employed in modern electrolyzers.
Also known in the art are graphite based electrodes, wherein the porous graphite base contains, either on the surface or in pores thereof, metals or metal compounds possessing electrocatalytical properties. Thus, known is an electrode comprising a current-conducting base of graphite with a coating consisting of a mixture of one or more oxides of the following film-forming metals: aluminium, titanium, tantalum, zirconium, niobium, bismuth and tungsten with one or more of the following metals: palladium, platinum, rhodium, iridium, ruthenium, osmium, gold, silver, iron, nickel, chromium, lead, copper, manganese; oxides of these metals, their nitrides, carbides, sulphides, as well as mixtures thereof (of USSR Inventor's Certificate No. 369923).
Known are also electrodes with an electroconducting (including graphite) base coated with oxides of metals of the platinum group added with oxides of non-noble metals such as tin (cf. FRG Application No. 2,710,802), .beta.-manganese dioxide (cf. FRG Application No. 2,636,447), cobalt oxides of the general formula Co.sub.3 O.sub.4 (cf. USSR Inventor's Certificate No. 492301).
These electrodes are considerably cheaper than those having a metal base and their wear during the initial operation period (generally about one month) is substantially lower than wear of a graphite anode impregnated with an electrochemically inert organic compound. However, with lapse of time, the process becomes occurring substantially totally on the graphite due to a broken contact "electrocatalytic compound-graphite" and the anode is subjected to wear in much the same manner as a non-treated graphite anode. This disadvantage of graphite anodes containing electrocatalytic compounds is responsible for the fact that said anodes have not obtained any practical application.
It is an object of the present invention to provide such a graphite electrode which would possess a long service life and could operate at commercial current density values.